They can be a tremendous help to certain horses. As a whole I think it's important to desensitize your horse to all things, including sounds. The more brave your horse is, the better, right? Plus, you might be in a situation some day where you don't have the ear plugs in and your horse will spook.
That being said, there are certain instances where the plugs are a gift from heaven and I love them! The pom pom (fuzzy) type plugs are ok. The BEST ones are the "Pomms", the squishy rubber balls. They don't fall out nearly as easily and they block out more sound. They are, however, $10 a pair. What they don't advertise is that they are exactly the same thing as those $1 a pack foam cat toy balls, they just aren't florescent colored! I know a ton of people who school in the cat toys (plus there's a better chance you won't forget to take them out when you see that lime green peeking out) and show in the dark Pomms.